It has been 10 years or so since Loma Linda for proton radiation. They used a full body mold.
When in the mold and on the system to which you (and the mold) are attached, they place a balloon up your rectum which apparently makes the target location on the prostate more defined. A dry run, if you will was done whereas the machine to which provides the radiation "locks" on the location of the cancer. So, every morning this was done for 10 weeks.

Usually at that time, there were two machines for prostate and several more for other patients undergoing proton radiation therapy. Loma Linda is a children's hospital and proton was use for children with brain cancer. That alone provides a proof source to the exactness of the procedure.

The waiting room for those of us waiting in the morning as we were in gowns (the back open) and slippers. So, I had a chance to speak briefly with perhaps 10 or 15 patients over my time.
Nearly all were doctors! That alone provided the ultimate proof source to my decision on what and where.

The "brotherhood of the balloon" was a well used phrase..those of us who used the balloon as mentioned above. No doubt other forms of exactness are used. so check around so you understand exactly what and how the procedure is administered.